JP2964861B2 - Stainless steel manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an argon oxygen decarburization furnace (A
OD furnace) and a method for producing stainless steel using hot metal, scrap, alloyed iron, and the like in a steelmaking furnace such as a converter having a double-blowing function.

[0002]

2. Description of the Related Art Cr is contained as a stainless component, for example.
When producing stainless steel with Cr: 11% or more, hot metal from a blast furnace is used and molten steel melted in an electric furnace.
(Hot metal) may be used, but in any case, as shown in the process diagram in Fig. 1, a pretreatment step 10 for oxidative dephosphorization, then ferrochrome is added, and an AOD furnace or Oxygen blowing in a furnace etc. to decarburize to the target carbon content while maintaining the [Cr] concentration in the molten metal14, and the chromium oxide generated at this time was reduced and desulfurized Thereafter, a reductive desulfurization finishing step 16 in which a ferroalloy is also charged for component adjustment is sequentially performed in the same or another type of processing furnace.

[0003] As described above, using an AOD furnace, a converter equipped with a double-blowing function, etc.
In the case of decarburizing a crude molten stainless steel), the oxidation of chromium inevitably proceeds in parallel, but since chromium is an expensive alloying element, its oxidation must be avoided as much as possible. However, since chromium has a strong affinity for oxygen, it is impossible to completely eliminate chromium oxidation. Therefore, chromium is usually present in decarburized slag in the form of chromium oxide.
It is contained around 10-60%.

In order to recover the oxidized chromium,
After completion of decarburization, chromium oxide in decarburized slag is reduced and recovered using expensive ferrosilicon and quick lime in a reduction period. In the reduction period,
Since the obtained slag becomes reducing, the desulfurization reaction also proceeds at the same time.

[0005]

However, the method of adding ferrosilicon or metallic silicon in the above reduction period has the following problems. (1) Large amounts of Si (often Si as ferrosilicon)
, The cost increases. (2) Since SiO ₂ is generated as a reaction product, a large amount of CaO is required to neutralize it, and as a result, a large amount of slag is generated. (3) Since the reduction reaction of chromium oxide with ferrosilicon is an exothermic reaction, the temperature rises and the slag is rich in fluidity, so that the refractory is eroded. For this reason, the furnace life is one-tenth or less in the refining of stainless steel as compared with ordinary carbon steel refining.

Therefore, it is considered that the above problem can be solved by using carbon as a reducing agent for reducing chromium oxide in slag, but adding carbon as a reducing agent at the end of decarburization is difficult. Such measures cannot be adopted because they cause carburization of the molten steel.

Conventionally, the following solutions have been proposed for such a problem. For example, JP-A-2
As shown in FIG. 2, the slag 26 containing chromium oxide generated in the decarburization refining step 24 is recycled in the smelting reduction step in a method comprising a smelting reduction step 20 → a desulfurization step 22 → a decarburization refining step 24 as shown in FIG. It has been proposed to use or recycle the chromium oxide-containing slag in a decarburization refining process. This method is intended for the smelting reduction of chromium ore,
This is a technology when the [Cr] concentration is as high as about 18%.

However, in this method, the chromium oxide in the slag must be reduced and recovered only by carbon in the molten metal having a high [Cr] concentration. In this case, the chromium oxide concentration in the slag and the chromium concentration in the molten metal are reduced. The value of the ratio (Cr ₂ O ₃ ) / [Cr] is defined to be a constant value when given conditions such as temperature, so (Cr ₂ O ₃ ) in the slag when [Cr] in the molten metal is high However, there is a problem that the reduction rate is low.

Thus, an object of the present invention is to improve the yield of Cr without using a large amount of metal such as Si or Al as a reducing agent, and furthermore, as disclosed in the method of JP-A-2-232312. It is another object of the present invention to provide a new method for producing stainless steel which has solved the problem that the reduction rate remains low only by reducing and recovering chromium oxide in slag with carbon in molten metal having a high [Cr] concentration. .

[0010]

Means for Solving the Problems Accordingly, the present inventors have
After examining various processes that do not have these problems,
The following process was reached. In other words, pretreatment (dephosphorization of molten metal that does not contain chromium),
The slag containing chromium oxide generated during decarburization of the ferrochrome-added molten stainless steel is dephosphorized from the next charge in the same steelmaking furnace (eg, AOD furnace, converter) or melted in an electric furnace. By reusing the molten metal during decarburization, it was found that chromium oxide can be reduced with the carbon in the molten metal or the carbon in the carbonaceous material to be added and chromium can be recovered in the molten metal. .

Therefore, according to the present invention, it is possible to omit the conventional operation in the reduction period using a large amount of ferrosilicon after the end of decarburization. However, desulfurization cannot be expected to the extent that the reduction period has disappeared, so that a separate desulfurization treatment is required after tapping.

Further, for the reduction of chromium at a higher concentration, the present invention provides a method for removing slag containing chromium oxide, which is generated when decarburizing a crude melt of stainless steel, preferably with a basicity of CaO / SiO ₂ 0.8. Under the conditions of 3.5 or less, the decarbonized pig iron or the electric furnace is recycled at the time of decarburization heating and blowing of the molten metal after the next charge in the same steelmaking furnace, and the carbon in the molten metal or the carbonaceous material added during decarburization is used. reduction of the chromium oxide in said slag in wood, after recovering the resulting chromium in the molten metal, the chromium oxide is noted remaining in the slag by addition of alloy containing shea Li <br/> con After reducing and recovering chromium in the molten metal,
This is a method for producing stainless steel in which debris is removed, ferrochrome is added so as to have a target chromium concentration in the final stage of decarburization refining, steel is removed after decarburization refining, and then desulfurization treatment is performed outside the furnace.

More specifically, the present invention relates to a method for producing a stainless steel including a decarburization step of decarburizing a crude molten metal to a target carbon content by oxygen blowing, wherein the precharged stainless steel is provided. The slag containing chromium oxide generated when decarburizing the crude molten metal is preferably of a basicity of CaO / SiO _2.
Under the condition of 0.8 to 3.5, recycle and reuse during decarburization heating and blowing of dephosphorized pig iron after the next charge in the same steelmaking furnace or decarburization heating and blowing of molten molten metal in the electric furnace. During the charcoal treatment, the chromium oxide in the slag is reduced by the carbon in the molten metal or the added carbonaceous material, and further contains silicon
After adding the alloy to be recovered and collecting the obtained chromium in the molten metal, removing the slag, adding ferrochrome so as to have a target chromium concentration in the final stage of the decarburization refining, and removing the steel after the decarburization refining,
Next, a desulfurization treatment is performed outside the furnace.

[0014]

Next, the operation of the present invention will be described more specifically with reference to the accompanying drawings. FIG. 3 is a process diagram of the method for producing stainless steel according to the present invention. In the present invention, a pretreatment step as in the prior art, that is, a dephosphorization step 10 of a chromium-free molten metal, a dephosphorization The furnace is demelted through a decarburization heating step 12 for raising the decarburization temperature of the crude molten metal and reducing chromium contained in the slag, a decarburization refining step 14 after the addition of ferrochrome, and finally an out-of-furnace desulfurization step. 19
To obtain a stainless steel.

Here, according to the present invention, the chromium oxide-containing slag 18 obtained in the decarburizing and refining step 14 of the molten stainless steel after the addition of ferrochrome is made of dephosphorized pig iron or an electric furnace molten crude molten metal containing almost no chromium. It is recycled and reduced in the decarburization heating step 12. That is, it is recycled in the decarburization heating process of the next charge.

In this respect, the prerequisite steps are different from the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-232312. That is, the method disclosed in Japanese Patent Application Laid-Open No. 2-232312 is used in the smelting reduction step of chromium ore, in which slag is recycled into the molten metal having a high chromium concentration of about 18%. The high chromium concentration greatly reduces the reduction yield of chromium in the slag.

As described above, according to the present invention, the slag containing chromium oxide generated during the decarburization and refining of the molten stainless steel after the addition of ferrochrome is replaced with the dephosphorized iron or the electric furnace molten molten metal after the next charge. By reducing the chromium oxide by using carbon or coke or other carbonaceous material in the molten metal by recycling during decarburization heating, ferrosilicon or metal required in the reduction period of the conventional method
The addition of Si can be omitted or the amount used can be reduced.

In the decarburization heating step 12, the slag containing chromium oxide produced during the decarburization and refining of the molten stainless steel before the pre-charging, that is, during the decarburization after the addition of ferrochrome, is recycled and used. or carbonaceous material to be added (e.g. coke) after the chromium is reduced chromium oxide with carbon in collected in a molten metal, by adding an alloy containing silicon, such as small amounts of ferrosilicon Furthermore,
The chromium oxide still remaining in the slag is reduced to recover the chromium in the molten metal, so that the yield of chromium recovery can be further improved. When ferrosilicon is added, better desulfurization can be expected.

Further, when recycling the slag containing chromium oxide generated during the decarburization and refining of the stainless steel crude molten steel, that is, the molten stainless steel, a carbonaceous material such as coke, char, and anthracite is added. By blowing oxygen, it becomes possible to supply heat necessary for reducing chromium oxide by burning carbon, and to supply carbon as a reducing agent necessary for reducing chromium oxide.

In the present invention, the slag containing chromium oxide generated during the decarburization and refining of the molten stainless steel is recycled and used to reduce the chromium oxide with carbon in the molten metal or carbon in the carbonaceous material to be added. When chromium is recovered in the molten metal, the slag basicity CaO / SiO ₂ at that time is adjusted to be in the range of 0.8 to 3.50 by adding quicklime, silica sand and the like.

If the basicity of the slag is less than 0.8, the forming of the slag becomes large, the operation becomes unstable, and the refractories are greatly worn away.
If it exceeds 50, the fluidity will deteriorate due to the rise in the melting point of the slag,
This is because the reduction rate of chromium oxide is significantly reduced. More preferably, the value of the basicity CaO / SiO ₂ is 1.0 to
3.0 is good.

The reason why desulfurization treatment is performed outside the furnace on the molten stainless steel produced by the method of the present invention is that the reduction period in the conventional method is omitted and desulfurization there cannot be expected. The method of the desulfurization treatment is not particularly limited. For example, injection of CaO-based flux into molten steel in a ladle may be considered. In the present invention, there are the following two methods for recycling the slag containing chromium oxide obtained in the decarburization refining process to the next charge.

After the decarburization is completed, only the molten steel is tapped, the slag is left in the furnace, and the next charge of dephosphorized iron or the electric furnace melt is added. After the decarburization is completed, both the molten steel and slag are discharged outside the furnace, and the next charge of dephosphorized iron or the electric furnace molten crude molten metal is charged, and then only the slag is recycled and charged into the furnace.

When the slag containing chromium oxide is recycled into dephosphorized pig iron or molten metal in an electric furnace and is blown, one of Ar, N ₂ , and O ₂ gases is used as a bottom blowing gas, or What is necessary is just to use two or more types of mixed gas properly according to the objective.

[0025]

EXAMPLES (Example 1) (Reference Example) 1400 ° C. de Rinzuku 90t the ([Cr] = 0.02%) was charged to the upper and lower blown converter, the Ar gas from the bottom tuyeres 45 Nm ³ / min, slag collected at the end of the decarburization refining period of the pre-charged crude stainless steel while blowing oxygen at 320 Nm ³ / min from the top blowing lance (hereinafter referred to as decarburization refining final slag. Composition:
T. Cr = 13%, T.C. Fe = 3%, CaO / SiO ₂ = 3.3, MgO
= 15%) 6100 kg, coke 7900 kg, quicklime 950 k
g, 1600 kg of silica sand and 530 kg of granular MgO were added and decarburization heating and blowing was performed for 35 minutes. The hot metal composition after the above treatment is [C]
= 4.3%, [Cr] = 0.77%, [S] = 0.012%, temperature is
The temperature was 1510 ° C., and the T.Cr in the slag was 1.5%.

Next, the slag was removed from the furnace, and the same decarburization refining was performed for 44 minutes while adding 29,000 kg of high-carbon ferrochrome. During this time, the top blown oxygen flow rate was 290 Nm ³ / min for the first 34 minutes, and 130 Nm 3 every 5 minutes thereafter.
³ / min, stepwise reduced to 80 Nm ³ / min. The bottom blown Ar flow rate was kept constant at 100 Nm ³ / min throughout the blowing period.

The composition of the molten steel after the above treatment was [C] = 0.05%,
[Cr] = 16.1%, [S] = 0.012%, temperature was 1710 ° C. The slag obtained at the end of decarburization refining obtained here was recycled for the decarburization heating process of dephosphorized pig iron. On the other hand, the molten steel in the ladle was tapped, and 300 kg of CaO was added to the molten steel in the ladle.
The -15% CaF ₂ flux was injected. This treatment reduces the [% S] of the molten steel to 0.002%,
The temperature reached 1610 ° C.

Example 2 91 tons of dephosphorized pig iron at 1300 ° C. ([Cr] =
0.02%) into the converter and Ar gas of 45Nm from the bottom tuyere
³ / min, while blowing 320 Nm ³ / min of oxygen from the top blowing lance, 6100 kg of decarburized refining slag, 7900 kg of coke, and 95 of quick lime of the crude stainless steel obtained in the previous charge
0 kg, 1600 kg of silica sand and 530 kg of granular MgO were added, and decarburization heating and blowing was performed for 35 minutes.

The hot metal composition after the above treatment was [C] = 4.2%,
[Cr] = 0.76%, [S] = 0.013%, the temperature was 1505 ° C., and T.Cr in the slag was 1.5%. Then, Fe-Si
290 kg was added to reduce the Cr ₂ O ₃ still remaining in the slag, (T. Cr) in the slag was 0.3%,
[% Cr] = 0.89 and [% S] = 0.005.

Next, the slag was removed from the furnace, and decarburization refining was performed for 44 minutes while adding 29,000 kg of high-carbon ferrochrome. During this time, the top blown oxygen flow was 29 for the first 34 minutes.
0 Nm ³ / min, then every 5 minutes 130 Nm ³ / min, 80 Nm ³ / mi
n and gradually decreased. The bottom blown Ar flow rate was kept constant at 100 Nm ³ / min throughout the blowing period.

The composition of the molten steel after the above treatment is [C] = 0.06%,
[Cr] = 16.3%, [S] = 0.005%, and the temperature was 1690.degree. The stainless molten steel was tapped to a ladle was blown on the CaO -40% CaF ₂ powder 300 kg molten steel in the ladle. This treatment reduces the [S] of the molten steel to 0.001% and raises the temperature to 1600%.
° C. Table 1 summarizes the results of Examples 1 and 2.

[0032]

[Table 1]

(Example 3) 90 tons of molten metal produced by an electric furnace
(Temperature 1300 ° C, composition [C] = 1.4%, [Cr] = 0.02%) was charged into the converter, and Ar gas was supplied at 45 Nm ³ / min from the bottom blow tuyere.
While blowing oxygen at 320 Nm ³ / min from the top blowing lance, 6100 kg of decarburized refining slag obtained at the previous charge, 7900 kg of coke, 950 kg of quicklime, 1600 kg of silica sand, granular Mg
530 kg of O 2 was added and decarburization heating and blowing was performed for 35 minutes.

The composition of the molten metal after the above treatment was [C] = 1.7%,
[Cr] = 0.77%, [S] = 0.016%, the temperature was 1600 ° C., and T. Cr in the slag was 1.7%. Then Fe-Si
285 kg was added to reduce the Cr ₂ O ₃ still remaining in the slag, (T. Cr) in the slag was 0.3%, [Cr] in the metal was 0.9%, and [S] was 0.004. %.

Next, ferrochrome was added in the same manner as in Example 2, and decarburization and refining of the molten stainless steel was carried out. The molten steel composition after the above treatment was [C] = 0.06%, [Cr] = 16.5%,
[S] = 0.004% and the temperature was 1695 ° C.

The molten steel is tapped into a ladle, and the molten steel in the ladle is removed.
The CaO -40% CaF ₂ powder of 300kg was blown over. This treatment reduced the [S] of the molten steel to 0.001%, and the temperature reached 1610 ° C.

Example 4 (Reference Example) 90 tons of dephosphorized pig iron ([Cr] = 1390 ° C to 1415 ° C )
0.02%) into the converter, and Ar 50%, O ₂
A 50% mixed gas of 45 Nm ³ / min and oxygen of 320 Nm ³ / min from the top blowing lance are blown in, while the decarburizing and refining slag of the crude stainless steel melt (composition: T.Cr = 13%, T.Fe = 3
%, CaO / SiO ₂ = 3.3, MgO = 15%) to 6000 kg, coke to 9000 kg, quicklime to 950 kg and granular MgO to 500 kg, and silica sand to a predetermined amount in the range of 500 kg to 4000 kg. Decarburization heating and blowing was performed for 45 minutes.

The hot metal temperature at the end of blowing is 1515 ° C. to 1535
C and the slag basicity was in the range of 0.65 to 2.3. Hot metal [C] at the end of blowing was 4.2
Was in the range of ~ 4.4%. FIG. 4 is a graph showing the relationship between the basicity of the slag and the refractory erosion rate at the end of blowing.

FIG. 4 shows that when the basicity of the slag is less than 0.8, the erosion rate of the refractory increases sharply. After this,
The slag was discharged outside the furnace, and the same ferrochrome addition, decarburization refining, and desulfurization treatment as in Example 1 were performed.

(Example 5) (Reference example) 90 tons of dephosphorized pig iron at a predetermined temperature in the range of 1380 ° C to 1420 ° C
Was charged with ([Cr] = 0.02%) in both the upper and lower blown converter, a bottom blown tuyeres than N ₂ gas 2 Nm ³ / min., Oxygen from the top lance blowing 250 Nm ³ / min, stainless steel Decarburization refining slag of crude molten metal (Composition: T.Cr = 13%, T.Fe =
3%, CaO / SiO ₂ = 3.3, MgO = 15%) 5900 kg, coke 1200 kg, quicklime 0-1000 kg and granular MgO 450
In addition to the kg, 0 to 1000 kg of silica sand was added and decarburization heating and blowing was performed for 33 minutes. In addition, the oxygen from the top blowing lance was reduced to 110 Nm ³ / min in 18 minutes during blowing, and further 23 minutes in
It was lowered to 70 Nm ³ / min. The metal temperature at the end of the heating and blowing of the dephosphorized iron was in the range of 1575 ° C to 1595 ° C, and the basicity of the slag was in the range of 1.5 to 4.5. The metal [C] at the end of the blowing was in the range of 0.15 to 0.25%.

FIG. 5 is a graph showing the relationship between the basicity of slag and the Cr recovery at the end of blowing. FIG. 5 shows that when the basicity of the slag exceeds 3.5, the Cr recovery rate sharply decreases. Thereafter, the slag was discharged out of the furnace, and
The same ferrochrome addition, decarburization refining, and desulfurization treatment were performed.

[0042]

According to the method of the present invention, it is possible to reduce chromium oxide in slag during dephosphorization of the next charge or decarburization of the molten metal in the electric furnace.
It is possible to omit the reduction period using ferrosilicon after completion of the conventional decarburization refining of the crude stainless steel melt, and it can be understood from this that the amount of ferrosilicon and quicklime used for basicity adjustment and the amount of generated slag can be reduced.

Further, the decarburized refining final slag after the addition of ferrochrome is recycled and used, and the chromium oxide contained in the slag is reduced to recover chromium in the molten steel.
By adjusting the basicity to 0.8 to 3.5, it is possible to suppress the erosion of the refractory of the steelmaking furnace and at the same time to improve the recovery rate of chromium. Therefore, in the method of the present invention, since the reduction period by ferrosilicon can be omitted, MgO-C brick which could not be used in the conventional method due to the problem of carburizing molten steel can be used. In other words, the life of the refractory can be extended as compared with the case where the conventional Magdro brick or Magcro brick is used.

[Brief description of the drawings]

FIG. 1 is a process diagram of a conventional example.

FIG. 2 is a process chart of another conventional example.

FIG. 3 is a process chart of a method for producing stainless steel according to the present invention.

FIG. 4 is a graph showing the relationship between slag basicity and refractory erosion rate.

FIG. 5 is a graph showing the relationship between slag basicity and Cr recovery.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masaki Miyata 4-33, Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries, Ltd. (56) References JP-A-6-73424 (JP, A) JP-A Sho 59-166611 (JP, A) JP-A-58-22318 (JP, A) JP-A-60-52505 (JP, A) JP-A-1-219118 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21C 5/28 C21C 5/36 C21C 1/02

Claims (4)

(57) [Claims] Claims 1. A slag containing chromium oxide generated during decarburization refining of a crude molten stainless steel, is heated at the same time as the next charge in the same steelmaking furnace, and is subjected to heating and blowing of dephosphorized iron or an electric furnace. The chromium oxide in the slag is reduced by the carbon in the molten metal or the added carbonaceous material, and an alloy containing silicon is added. After removing the slag, at the end of the decarburization refining, ferrochrome is added so that the target chromium concentration is obtained, and the steel is decarburized and refined, and then subjected to desulfurization treatment outside the furnace. Production method. 2. A method for producing stainless steel, comprising a decarburization refining step of decarburizing a crude molten metal to a target carbon content by oxygen blowing, wherein the pre-charged stainless steel crude molten metal is decarburized and refined. The slag containing chromium oxide generated at that time is recycled during decarburization heating and blowing of dephosphorized iron after the next charge in the same steelmaking furnace or during decarburization heating and blowing of molten molten metal in the electric furnace. Reducing the chromium oxide in the slag with the carbon in the melt or the carbonaceous material added during the decarburization process,
Further, an alloy containing silicon is added, and the obtained chromium is recovered in the molten metal, and then the slag is removed.At the end of the decarburization refining, ferrochrome is added so as to have a target chromium concentration, and then the steel is produced after the decarburization refining. And then performing desulfurization treatment outside the furnace. 3. The method for producing stainless steel according to claim 1, wherein said slag containing chromium oxide has a basicity CaO / SiO ₂ of 0.8 to 3.5. 4. When recycling slag containing chromium oxide generated during decarburization of a stainless steel crude molten metal, heat required for reduction of the chromium oxide is generated by burning carbonaceous material added to the molten metal. The method for producing stainless steel according to any one of claims 1 to 3, wherein the stainless steel is supplied.